Viruses

1. Viruses

2. Viruses • Nonliving particles • Very small (1/2 to 1/100 of a bacterial cell) • Do not perform respiration, grow, or develop • Are able to replicate (only with the help of living cells) • Host cell—a cell where a virus replicates • Bacteriophage (phage)—virus that infects a bacterium

3. 0.5 m T4 bacteriophage infecting an E. coli cell

4. Virus Bacterium Animalcell Animal cell nucleus 0.25 m Comparing the size of a virus, a bacterium, and an animal cell

5. Viral Structure • 2 main parts: • inner core of nucleic acid (DNA or RNA) • instructions for making copies of the virus • outer coat of protein (capsid) • determines shape of virus (which cells & how cells are infected) • polyhedral • helical • envelope with projections • classic phage shape

6. Capsomereof capsid Membranousenvelope RNA Capsomere DNA Head Capsid Tail sheath DNA RNA Tail fiber Glycoprotein Glycoprotein 80  225 nm 18  250 mm 80–200 nm (diameter) 70–90 nm (diameter) 50 nm 20 nm 50 nm 50 nm (d) Bacteriophage T4 (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses Viral structure

7. Infection by tobacco mosaic virus (TMV)

8. Attachment to Host Cell • Order of events: • virus recognizes host cell • virus attaches to receptor site on membrane of host cell • Receptor site on host matches with viral proteins (like a puzzle) • virus enters host cell • virus replicates inside host cell

9. Attachment is Specific • viruses have specifically shaped attachment proteins • each virus infects only certain types of cells • most are species specific • Smallpox, polio, measles—affects only humans • although some are not • West Nile virus—mosquitoes, birds, humans, horses • some are cell-type specific • polio—affects intestine & nerve cells

10. VIRUS Entry into cell and uncoating of DNA DNA Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell Simplified viral reproductive cycle

11. Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell. Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed. 1 2 4 3 5 Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles. Phage assembly Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell. Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms. Head Tail fibers Tails Lytic cycle of phage T4, a virulent phage

12. Lytic vs Lysogenic • Lytic cycle (virulent phage) • Release of virus bursts and kills host cell (lysis) • Lysogenic cycle (temperate phage) • Viral DNA integrates into host genome (provirus) • Can be transmitted to daughter cells • Can initiate lytic cycle in response to environmental signal (stress)

13. Phage DNA The phage attaches to a host cell and injects its DNA. Many cell divisions produce a large population of bacteria infected with the prophage. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle Lysogenic cycle Certain factors determine whether The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Prophage/Provirus or New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome,becoming a prophage (provirus). The lytic and lysogenic cycles of phage , a temperate phage

14. Glycoprotein Viral envelope Capsid Reversetranscriptase RNA(two identicalstrands) The structure of HIV, the retrovirus that causes AIDS

15. The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 1 HIV Membrane of white blood cell 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis ofa second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNAhybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. mRNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 7 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 9 New viruses bud off from the host cell. New HIV leaving a cell The reproductive cycle of HIV, a retrovirus

16. Complete the Following Venn Diagram. Describe in detail similarities and differences, give examples.

17. Viral Diseases • Viruses are pathogenic and therefore disease causing. • Some viral diseases: • HIV • Influenza • Colds • Polio • Rabies • Smallpox • Hepatitis

18. HIV • HIV is a retrovirus and can be spread by sexual contact, contaminated blood, or contaminated needles. • HIV affects the immune system by destroying helper T cells. • Eventually HIV will lead to AIDS, which is a disease in which a person’s immune system is unable to defend against infections.

19. Viral Diseases • Some viral diseases can also cause cancer. • Hepatitis B = liver cancer • Epstein-Barr virus = Burkitt’s lymphoma • Human papilloma virus (HPV) = cervial cancer